1. Field of the Invention
The present invention relates generally to secondary batteries, and more particularly, to an improvement of the performance of a lithium battery with a negative electrode containing graphite particles which can cause intercalation and de-intercalation of lithium ions.
2. Description of the Related Art
As miniaturization and power saving have proceeded in the field of electronics, secondary batteries using alkali metals such as lithium have attracted attention. A battery using alkali metal alone such as lithium for a negative electrode is encountered with a problem of short circuit in the battery by repetition of charge/discharge. More specifically, repetition of charge/discharge repeats dissolution and precipitation of the alkali metal, and a dendrite of the alkali metal grows on the surface of the negative electrode. The dendrite grows penetrating through the separator between the negative electrode and the positive electrode, and comes into contact with the positive electrode to form a short circuit.
Use of an alkali metal alloy for the negative electrode of a secondary battery instead of an alkali metal decreases such growth of dendrite and improves charge/discharge cycle characteristic. Use of an alkali metal alloy for the negative electrode however cannot completely prevent the growth of dendrite and there is still a possibility of short circuit in the battery.
In recent years, a negative electrode of carbon or a conductive organic polymer material has been developed taking advantage of insertion and desertion of alkali metal ions rather than metal dissolution, deposition, diffusion in solid or the like as practiced in the case of an alkali metal or an alloy thereof. In such a negative electrode of carbon or a conductive polymer material, dendrites which have been grown in the case of the negative electrode of an alkali metal or an alloy thereof are prevented and the problem of short circuit in the battery has been solved.
Carbon which is a chemically stable substance and can be doped with either an electron-donor element or an electron-acceptor element is a preferable material for an electrode for a battery.
When carbon is used for an active material for a negative electrode, lithium can be intercalated between layers of carbon in 1 lithium atom per 6 carbon atoms at maximum, in other words at most LiC.sub.6. At the upper limit, theoretical capacity only by a reaction between carbon and lithium is 372 mAh/g (unit weight of carbon).
Carbon can take various forms from amorphous carbon to graphite. Sizes and arrangements of hexagonal nets of carbon atoms vary depending upon starting materials and manufacturing processes. Use of carbon materials (except graphite) as active materials for negative electrodes are disclosed for example in Japanese Patent Laying-Open Nos. 62-90863, 62-122066, 63-213267, 1-204361, 2-82466, 3-252053, 3-285273, and 3-289068. None of carbon materials disclosed in these prior art documents can achieve the theoretical capacity described above. Even among carbon materials with relatively large charge/discharge capacity, some have their potentials linearly changed at a considerable gradient and do not have enough capacity in the range of voltage used in a battery assembled in practice. More specifically, the carbon materials disclosed in these prior art documents are not satisfactory as a material for manufacturing a negative electrode for a battery with sufficient charge/discharge capacity.
Fong et al reported in J. Electrochem. Soc., Vol.137,1990, pp. 2009-2013 that discharge capacity corresponding to the theoretical capacity described above is obtained by use of graphite material as an active material for a negative electrode. This report, however, concerns only small discharge current and does not directly apply to batteries in practice. Japanese Patent Laying-Open Nos. 4-112455, 4-115457, 4-115458, 4-237971 and 5-28996 discloses use of graphite materials as active materials for negative electrodes, but none of these graphite electrodes reaches the theoretical capacity described above and is not satisfactory for manufacturing a battery with high capacity.
In Japanese Patent Laying-Open No. 3-216960, a lithium layer is formed on the surface of a porous carbon material such that pores are not occluded and thus a secondary battery permitting discharge of large current and having improved cycle life and safety is provided. In Japanese Patent Laying-Open No. 4-39864, use of a negative electrode immersed with a metal, which can form an alloy with lithium or an alloy including lithium, inside pores in the carbon material provides a secondary battery with large capacity and improve charge/discharge cycle life and self-discharge characteristic. In these batteries, however, lithium must be treated in an inactive atmosphere which complexes the process of manufacturing electrodes and pushes up cost for the batteries.
While attempts have been made to improve battery cycle life and discharge capacity at large current after storage at a high temperature, for example, by use of a negative electrode of a carbon material coated with a metal (such as nickel and copper) in Japanese Patent Laying-Open No. 4-184863 and by use of a negative electrode of a compound of carbon and at least a metal (one kind of metal such as Ni, Al, Cu or Fe which does not form an alloy with lithium) in Japanese Patent Laying-Open No. 4-259764, much increase of capacity of negative electrodes cannot be expected.
In Japanese Patent Laying-Open No. 5-21065, in order to improve the cycle life of a battery having a chalcogen compound as a main material for a negative electrode which allows dope and dedope of lithium ions, use of a mixture produced by adding a carbon material to a chalcogen compound for a negative electrode is attempted. A reaction of dope or dedope of lithium ions occurs when mean voltage to a lithium reference electrode is about 1 V, and therefore high energy density cannot be provided for such low charge/discharge voltage.